The initial onset of mania, and by definition bipolar I disorder, most frequently occurs during childhood and adolescence. Bipolar disorder is associated with significant psychosocial morbidity as well as increased rates of obesity, metabolic syndrome, and associated cardiovascular risk factors, and outcomes data indicate excess premature mortality attributable in part to cardiometabolic-related disorders. Although second generation antipsychotic (SGA) medications are efficacious for the treatment of acute mania in bipolar youth, they frequently lead to excessive weight gain and obesity and precipitate and/or exacerbate cardiometabolic risk factors including elevated triglyceride levels. This is a significant concern because SGA-induced cardiometabolic symptoms and weight gain increase risk of treatment discontinuation and relapse, more frequent suicide attempts, and poses substantial long-term health risks in adulthood. There is therefore an urgent and unmet public health need to identify modifiable risk and protective factors for SGA-induced cardiometabolic symptoms and weight gain in pediatric and adolescent bipolar patients to guide ameliorative and preventative strategies. Emerging translational evidence suggests that long-chain omega-3 (LCn-3) fatty acid deficiency exhibited by first-episode adolescent manic patients may represent a modifiable risk factor for SGA-induced cardiometabolic symptoms and weight gain. Recent evidence also suggests that stearoyl-CoA desaturase, a lipogenic enzyme implicated in hypertriglyceridemia, insulin resistance, and obesity, may mediate the detrimental effects of SGA medications as well as the beneficial effects of LCn-3 fatty acids. We believe that extant evidence and our preliminary data supports the hypothesis that low LCn-3 fatty acid status is a modifiable risk factor for SGA-induced weight gain and cardiometabolic risk in pediatric and adolescent bipolar patients. Moreover, we hypothesize that SGA-induced weight gain and cardiometabolic risk are mediated through elevations in stearoyl-CoA desaturase activity, and that increasing LCn-3 fatty acids status through dietary supplementation will attenuate the progression of cardiometabolic symptoms and weight gain by reducing stearoyl-CoA desaturase activity. To evaluate these hypotheses, we propose a two-phase study design to first prospectively evaluate LCn-3 fatty acid status as a risk factor for the emergence of SGA-induced weight gain and cardiometabolic symptoms in first-episode bipolar youth (Phase I), and a 24-week randomized placebo-controlled double-blind LCn-3 fatty acid intervention trial to determine whether increasing LCn-3 fatty acid status can attenuate the progression of cardiometabolic symptoms and weight gain in SGA-treated patients (Phase II). It is anticipated that the data generated by this study will provide novel prospective evidence that low LCn-3 fatty acid status is a modifiable risk factor for SGA-induced adverse cardiometabolic effects and weight gain in adolescent bipolar patients in support of future large-scale primary prevention studies.